This invention relates to melting furnaces and particularly to a forehearth in a glass melting furnace.
In such a furnace, the heat within the furnace and within the forehearth is produced by Joule effect heating. Electrodes placed in the forehearth are energized by a source of AC power and a current flows through the electrodes and through the melted material between the electrodes.
A common problem within the forehearth is to produce heating while minimizing extraneous cross firing and undesirable current paths between separate groups of opposed electrodes. These separate groups of opposed electrodes are each connected to separate phase combinations of a multiphase power supply or to separate power supplies and are arranged to produce a current through the molten material in the forehearth and to heat the molten material by Joule effect heating. It is of maximum advantage to contain the current between a pair of opposed electrodes within an electrode set, so that substantially all the current flows between the pair of opposed electrodes and none flows to a separate set of electrodes connected to a different phase combination of the power supply, or to a separate power supply, through an extraneous firing path.
However, where the adjacent electrode groups of separate electrode sets are connected to different phase combinations of a multiphase power supply, the phase difference between the adjacent groups will cause some flow of current between the groups. Such is the case when separate transformers are used, each connected to different phase combinations, or separate supplies are used.
The prior art contains many examples of forehearth heating. Such is shown in U.S. Pat. Nos. 3,326,655, 4,029,488, as well as 3,400,204. Additionally, U.S. Pat. No. 4,029,488 shows underglass forehearth electrodes, as does U.S. Pat. No. 4,118,215.
The prior art also shows U.S. Pat. No. 3,961,126 which teaches an electric furnace for heating molten glass powered from a three phase source, and U.S. Pat. No. 3,985,944 showing a 60.degree. phase angle between adjacent electrodes in an electric furnace. Other prior art patents are U.S. Pat. Nos. 3,098,111, 3,182,112, 2,350,734 and 2,761,890.
However, none of the prior art patents teaches the arrangement of this invention in a forehearth wherein the forehearth is electrically heated by opposed electrode groups disposed along the forehearth and with undesirable cross firing limited between adjacent groups connected to different power supplies and not part of the desired firing path.